1. Field of the Invention
The present invention relates to a technique of discriminating a subject area of a radiation image from a background area exposed directly to radiation.
2. Description of the Related Art
In medical applications, digital image processing apparatuses capable of outputting digital radiation image data are becoming popular. In digital image processing apparatuses, techniques of image processing are essentially important. Of various image processing techniques for radiation image data, a gray-level conversion process is of particular importance to convert radiation image data so as to have proper pixel levels (brightness) and proper contrast that allow diagnosis to be easily performed.
In diagnosis using radiation image data, an entire image is not always necessary. On the contrary, in many cases, information is necessary as to a part of an image. Therefore, an unnecessary area is generally deleted from a total image, and a gray-level conversion curve is determined based on a statistical characteristic of the remaining necessary area of the image.
More specifically, for example, a subject area is extracted as shown in FIG. 6 so as not to include a background area (also called a directly-exposed area) in which a radiation directly reaches a sensor without passing though a subject, and the gray-level conversion curve is determined such that the range from a minimum value to a maximum value of the subject area falls within the range from a minimum density 0.2 D to a maximum density 3.2 D.
It is troublesome for a user to define a threshold value by which to discriminate between a subject area and a background area. To avoid this problem, many methods of automatically determining the threshold value are known. For example, in a method disclosed in “A Threshold Selection Method from Gray-Level Histograms” (N. Otsu, IEEE Transactions on Systems, Man, and Cybernetics, Vol. 9, No. 1, pp. 62-66, 1979), an image is divided into two classes, and the interclass variance is calculated. When the interclass variance has a maximum value for a particular pixel value, this pixel value is determined as the threshold value by which to separate the subject area and background area from each other.
In a method disclosed in Japanese Patent Publication No. 03-022968, a frequency threshold value is determined depending on an image-captured part of a body from a histogram of an entire image, and a minimum value and a maximum value of a particular image area of interest are determined based on the frequency threshold value.
In a method disclosed in Japanese Patent Laid-Open No. 61-245281, an entire image is divided into a regular mesh and the variance of pixel values is calculated for each element of the mesh. Mesh elements having a variance smaller than a properly determined threshold value are determined to be in a background area.
In the above-described method disclosed in Japanese Patent Publication No. 03-022968, the background area is detected by comparing the variances of the respective mesh elements with the properly determined threshold value. However, a specific method of determining the threshold value is not disclosed in Japanese Patent Publication No. 03-022968. In a case where a subject area has a pixel value variance with a small value similar to that of a background area, there is a possibility that the subject area is incorrectly recognized as a background area. Thus, this technique has a problem that high-accuracy detection of a background area is difficult.